1. Field of the Invention
The present invention relates to a process for heat-degreasing ceramic molded bodies containing an organic binder mainly comprising paraffin wax, wherein the organic binder is removed in a short period of time without forming cracks or deformation.
2. Description of the Prior Art
In the case where ceramic shaped bodies, such as turbocharger rotors, turbine rotors or the like, of intricate shape, are molded by injection molding or cast molding, a ceramic starting powder is incorporated with a large quantity of an organic binder, such as a thermoplastic resin, wax or the like, in order to improve moldability and then molded into ceramic shaped bodies. The shaped bodies containing a large quantity of the organic binder require a degreasing step to remove the organic binder therefrom by means of thermal decomposition, dissolution with an organic solvent, or the like, prior to sintering.
The heat-degreasing is a method for removing organic binders by thermal decomposition and gasification thereof. However, build-up of stress due to gases or heat of reaction evolving during the thermal decomposition of the binders causes cracks or deformation. Particularly, this tendency is remarkable in thick molded bodies or intricately shaped articles, such as turbocharger rotors or the like. Therefore, it has a disadvantage such that the temperature must be elevated very slowly, so that the degreasing usually requires 20-30 days.
Japanese Patent Application Laid-open No. 62-78,165 discloses a degreasing process wherein a first degreasing step is conducted at a temperature lower than the decomposition temperature of the binder in an oxidizing atmosphere and then a second degreasing step is conducted at the decomposition temperature or higher of the binder in a non-oxidizing atmosphere. However, this process has difficulties such that the degreasing time is not so shortened as a whole because the molded bodies which have been cooled down are taken out and then heated again, since semi-degreased molded bodies are readily damaged in handling, and since a non-oxidizing atmosphere is required, and the like.